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At present, carbon dioxide is considered the largest contributor among greenhouse gases. This review covers the current state of problem of carbon dioxide emissions from industrial and combustion processes, the principle of photocatalysis, existing literature related to photocatalytic CO2 reduction over TiO2 based catalysts and the effects of important parameters on the process performance including light wavelength and intensity, type of reductant, metal-modified surface, temperature and pressure.

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TiO/g-CN photocatalysts with the ratio of TiO to g-CN ranging from 0.3/1 to 2/1 were prepared by simple mechanical mixing of pure g-CN and commercial TiO Evonik P25. All the nanocomposites were characterized by X-ray powder diffraction, UV-vis diffuse reflectance spectroscopy, photoluminescence, X-ray photoelectron spectroscopy, Raman spectroscopy, infrared spectroscopy, transmission electron microscopy, photoelectrochemical measurements, and nitrogen physisorption. The prepared mixtures along with pure TiO and g-CN were tested for the photocatalytic reduction of carbon dioxide and photocatalytic decomposition of nitrous oxide. The pure g-CN exhibited the lowest photocatalytic activity in both cases, pointing to a very high recombination rate of charge carriers. On the other hand, the most active photocatalyst toward all the products was (0.3/1)TiO/g-CN. The highest activity is achieved by combination of a number of factors: (i) specific surface area, (ii) adsorption edge energy, (iii) crystallite size, and (iv) efficient separation of the charge carriers, where the efficient charge separation is the most decisive parameter.